Optical instruments and in particular endoscopic instruments for medical and non-medical purposes are in general fluid-tight systems. For a series of reasons, however, there is the possibility that moisture can penetrate the housing, which can result in misting of the optical systems that restricts the view. Problems can arise, for example, even during manufacture of the optical instruments in normal atmosphere, if the moisture residue of the atmospheric air precipitates inside the housing. In addition, moisture can penetrate through minor leaks at connecting points at which the instrument can be taken apart for purposes of maintenance, repair or assembly. An additional strong impact, in particular of medical optical instruments, is constituted by cleaning by means of autoclaving, in which the instrument under variable pressure is exposed to superheated steam at approximately 140 degrees C. This temperature impact can result in minute leaks which, again, can allow moisture to penetrate the housing.
To avoid misting problems of this type from moisture precipitating on the optical system, a known method with optical instruments is to install a hygroscopic substance in the housing that binds the moisture accumulating in the housing internal space before it precipitates on the at least one optical system.
Thus it is a known practice, for example, to install the hygroscopic substance in the housing in loose form. This has the disadvantage, however, that in moving the instrument, noises occur and in addition, owing to the movement, abrasion of the hygroscopic substance is generated that can settle on the optical systems as dust.
Another known practice, from EP 0 916 106 B1, is to install the hygroscopic substance in the form of prefabricated moulded bodies, such as spheres or small rods for example, in a recess in the eyecup and to place it in stationary manner in this recess by means of a fixer element.
This known drying agent arrangement has proven itself completely in the art, but the arrangement of the hygroscopic substance in a housing recess allows use of only a small quantity of drying agent and in addition requires a special configuration of the eyecup.
A generic drying agent arrangement is known, for example, from DE 103 44 109 A1. With this known arrangement, the hygroscopic substance can be configured, for example, in spherical form, such that these spherical moulded shapes are disposed in a cage for stationary fastening.